Silicone rubber sponges are generally manufactured by adding a crosslinking agent, foaming agent or the like to a silicone rubber compound, milling the mixture, and then extruding the mixture through an extruder whereupon it is foamed and cured in a continuous line by way of atmospheric hot air vulcanization (HAV) or other conventional processes. Commonly used crosslinking agents include acyl peroxides such as benzoyl peroxide, 2,4-dichlorobenzoyl peroxide, p-chlorobenzoyl peroxide, and p-methylbenzoyl peroxide, alone or combined with other peroxides such as dicumyl peroxide, 2,5-dimethylbis(2,5-tert-butyl peroxy)hexane, ditert-butyl peroxide, and tert-butyl perbenzoate. Included in the foaming agents are azobisisobutyronitrile, dinitropentamethylenetetramine, p,p'-oxybis(benzenesulfonyl hydrazide), N,N'-dinitroso-N,N'-dimethylterephthalamide, and azodicarbonamide.
Some foaming agents like azobisisobutyronitrile and dinitropentamethylenetetramine leave decomposition residues which are detrimental to the human body from a safe and hygienic aspect. Also, acyl peroxide cross-linking agents such as benzoyl peroxide, 2,4-dichlorobenzoyl peroxide, p-methylbenzoyl peroxide leave decomposition residues which are more or less detrimental to the human body. Formation of such decomposition residues is undesirable for the maintenance of a working environment, and removal of decomposition residues requires a long term of heating. In addition, sponges accompanied by a hygienic problem due to the use of these foaming and crosslinking agents are not recommended for utility in food and cosmetic fields.
In contrast, azodicarbonamide and p,p'-oxybis(benzenesulfonyl hydrazide) are known as foaming agents having minimal influence on the human body. It is also known that hygienically most preferred sponges are formed by cross-linking through platinum-catalyzed addition reaction between alkenyl and SiH groups. It is thus desired to produce silicone rubber sponges by using these foaming agents and the addition curing reaction route.
Preparation of silicone rubber sponges by way of addition reaction is disclosed in Japanese Patent Publication (JP-B) No. 43294/1972 whereas it is not indicated to use azodicarbonamide and p,p'-oxybis(benzenesulfonyl hydrazide) as foaming agents. JP-B 1459/1978 discloses a process for foaming and curing a silicone compound while partially effecting addition reaction, but it is not indicated to use azodicarbonamide and p,p'-oxybis(benzenesulfonyl hydrazide) as foaming agents. The latter process requires an extra step of aging for allowing addition reaction to take place partially.
Japanese Patent Application Kokai (JP-A) No. 223034/1986 discloses a polydiorganosiloxane based sponge which is prepared from a composition comprising a vinyl-containing polydimethylsiloxane, a liquid foaming agent which is gaseous at 25.degree. C. under atmospheric pressure, and a platinum or rhodium catalyst. The polydimethylsiloxane composition is cured through catalytic hydrosilation and foamed at the same time. Undesirably, the liquid foaming agent is difficult to handle and expensive.
JP-B 10180/1988 discloses a foamable silicone rubber composition comprising a polydiorganosiloxane having a viscosity of up to 100,000 centistokes, a polyorganohydrogensiloxane, and p,p'-oxybis(benzenesulfonyl hydrozide) as a foaming agent. A platinum catalyst catalyzes addition reaction between an alkenyl group of the polydiorganosiloxane and a SiH group of the polyorganohydrogensiloxane. This curing, coupled with concurrent foaming, results in a sponge. It is described that if azodicarbonamide and similar foaming agents are used instead, sponges are formed, but without curing, and the sponges lack strength. Curing does not take place probably because premature decomposition of the foaming agent occurs before the onset of addition curing reaction.
As mentioned above, an acceptable foamable silicone rubber composition of the addition reaction curing type using an azodicarbonamide foaming agent is not available up to the present.